Mice that repeatedly win fights are more likely to seek out further aggressive confrontations, while also displaying an increase in anxiety, according to a new study in the journal Frontiers in Neuroscience. These behavioral changes appear to be driven by structural and functional alterations in the mice’s brains, with the number of neurons in a particular region rising as a result of positive combat experiences.

To examine the role of aggression on behavioral and neurological development, the researchers devised an experiment whereby pairs of male mice were separated from one another by a partition that prevented physical contact but allowed the animals to smell, see, and hear each other. After a certain period, these partitions were lifted, invariably leading to fights between the mice.

After several weeks, the researchers placed the mice into groups of "winners" or "losers," depending on their fighting success.

They then removed half of the "winners" from the experiment, depriving them of aggressive confrontations for two weeks, while allowing the rest to continue fighting. Observing the behavioral changes of the mice, the study authors note that "winner" mice tended to become increasingly aggressive, and would spend more time close to the partition than their "loser" counterparts. Once the partitions were removed, the "winners" showed a greater desire to start a fight, and waited less time to attack their partner than they had at the start of the experiment.

Interestingly, even after being deprived of aggression for 14 days, "winner" mice still maintained this increased propensity for violence, and were notably more eager than "losers" to start fights once they were re-introduced to the experiment.

Perhaps even more intriguingly, these victorious mice appeared to become increasingly anxious as they fought more. For instance, when placed in a specially designed maze with open and covered areas, they began to show an ever-increasing propensity to hide in the shadows.

In an attempt to explain this behavior, the researchers later removed the brains of the mice and counted the number of neurons the hippocampus and the amygdala. They focused on these brain regions as they are both known to play a role in memory formation, emotional regulation and fear response. As such, the researchers suspected that repeated violent experiences could somehow generate changes to these regions.

^{Exactly how an increase in neurogenisis in the hippocampus results in more aggressive behavior is not fully understood. Andrej Vodolazhskyi/Shutterstock}

Results showed that the victorious mice had greater numbers of young neurons in the hippocampus, suggesting that their experiences had somehow triggered an increase in neurogenesis – the formation of new nerve cells as a result of cell division – in this part of the brain. Why this increases aggressive tendencies is not fully known, although previous studies have shown that mice with a naturally greater propensity for violence tend to have higher levels of neuronal cell division in a part of the hippocampus called the dentate gyrus.

Furthermore, levels of a protein called c-fos were found to be lowered in the amygdala in "winner" mice. Since this protein is an indicator of neuronal activity, the researchers conclude that repeatedly winning fights had somehow dampened the level of activity in the amygdala. Since this region is involved in the modification of anxiety responses, this finding may well explain the heightened anxiety seen in these mice.

Exactly how these changes to the structure and function of the hippocampus and amygdala occur following a successful fight is still something of a mystery, as is the mechanism by which these effects stimulate further aggressive behavior. However, having identified a correlation between aggression and neurological development, the study authors hope their work provides a platform for future research into treating behavioral disorders.